But the study's methodology has been criticized as flawed by outside experts. And the salvo seems to have done something President Obama had been unable to accomplish: unite fractious Democrats in support of the legislation.

Senator Kerry, who has criticized the lack of a public-run insurance option in the bill, said the insurers' action showed why a public option is necessary. And he painted the industry as an obstacle to reform.

"If you're not part of the solution, you're part of the problem," he said.

The AHIP study took Capitol Hill and the White House by surprise. The Obama administration thought it had worked out a deal with the insurers in which they would support the healthcare reform effort.

The insurance study, produced by PricewaterhouseCoopers, focuses on one irrefutable fact: The current effort is a leap into the fiscal unknown. No one really knows exactly what will happen to US spending on healthcare if sweeping reform legislation gets passed.

Furthermore, changes made to the Senate Finance bill in recent weeks have decreased the number of new customers the insurance companies would gain. For instance, senators have voted to lessen the penalties for those who choose to ignore the bill's mandate that all Americans get insurance coverage.

But the legislation would still prevent insurers from denying coverage to those with preexisting medical conditions.

Caught in this vise, insurers would have no choice but to raise the price of coverage, according to the AHIP report. Between 2010 and 2019, the typical US family would pay $20,700 more in premiums than it would have otherwise, says the study.

Critics say this figure is an exaggeration. The AHIP study does not take into account the effect of government subsidies intended to help lower- and middle-income Americans purchase health insurance, which are also part of the Senate legislation. Nor does it assume any increase in efficiencies in the health-insurance market due to the formation of centralized exchanges where individuals can shop for the best insurance deals.

Taking these things into account, MIT economist Jonathan Gruber says that the Senate Finance bill would actually make insurance less expensive for individuals and families who purchase coverage on their own.

"The [Senate Finance bill] will deliver savings ranging from several hundred dollars for the youngest consumers to over $8,500 for families," writes Professor Gruber in his analysis.